Neurons in the spinal cord dorsal horn of monkeys and cats, especially neurons in lamina I and the substantia gelatinosa (SG), have been functionally characterized and intracellulary stained with horseradish peroxidase. The dendritic arbors and axonal extensions of these functionally identified neurons have been examined at the light microscope level. Wide dynamic range neurons, responsive to innocuous stimuli but activated maximally by noxious stimuli are found in layers I-V of the dorsal horn and have more extensive arbors than neurons in the same layers activated only by noxious stimuli (nociceptive-specific). The cytomorphological analysis of these functionally characterized neurons will soon be extended to the level of resolution of the electron microscope.